Radar mounted on helicopter

ABSTRACT

A radar antenna is fixedly disposed within a radar dome secured to a helicopter on the rotor head so as to have a maximum directive gain between selected adjacent two of blades radially extending at predetermined equal angular intervals from the rotor head. The antenna is electrically connected to a transmitter/receiver unit through a transmission line extending through a rotor shaft and a rotary joint disposed at the lower end of the shaft. In order to provide a dual antenna system or to form a combined primary and secondary radar, another antenna may be similarly provided on the helicopter but its maximum directive gain is directed between another selected pair of adjacent blades.

United States Patent Kondoh et a1.

Primary ExaminerE1i Lieberman Attorney, Agent, or Firm-Robert E. Burns; Emmanuel J. Lobato; Bruce L. Adams A radar antenna is fixedly disposed within a radar dome secured to a helicopter on the rotor head so as to have a maximum directive gain between selected adjacent two of blades radially extending at predetermined equal angular intervals from the rotor head. The antenna is electrically connected to a transmitter/receiver unit through a transmission line extending through a rotor shaft and a rotary joint disposed at the lower end of the shaft. lnorder to provide a dual antenna system or to form a combined primary and secondary radar, another antenna may be similarly provided on the helicopter but its maximum directive gain is directed between another selected pair of adjacent blades.

ABSTRACT 6 Claims, 14 Drawing Figures 1 RADAR MOUNTED ON HELICOPTER [75] Inventors: Teruo Kondoh; Koki Nakatsuka,

both of Amagasaki, Japan [73] Assignee: Mitsubishi D enki Kabushiki Kaisha,

Japan [22] Filed: July 13, 1972 [21] App1.No.: 271,526

[52] US. Cl 343/705; 343/872 [51] Int. Cl. H01q 1/28 [58] Field of Search 342/705, 708, 872

[56] References Cited UNITED STATES PATENTS 2,462,102 2/1949 lstyan 343/705 2,984,834 5/1961 Howard 343/705 3,045,236 7/1962 Colman ct a1. 343/705 3,390,393 6/1968 Upton 343/708 3,543.271 11/1970 Scheel 343/911 L 3,701,157 10/1972 Uhrig .1 343/708 3,766,561 10/1973 Johnson 343/705 SYNC RESOLVER -72 CONTROL 6 UNIT PATENTEDJUL 2 2 ms SHEET FIG. I

R 4 Cw L YO SS 8 E 7 R -72 CONTROL "QFIG. 2

" 3,896,446 PATENTED JUL 2 2 I975 SEEN 2 FIG. 3

PATENTEDJUL22 ms SHEET FIG. 6

"RESOLVER I Q L SYNC.

I CONTROL FIG. 7

PATENTEDJUL 2 2 ms SHEET FIG. 8

FIG. 9

FIG/IO PATENTEDJUL22 ms 3, 896,446

SHEET 6 FIG. l3

4o\ F. 20 I 1 1 -1 l l H I i L 39 38 W90 20 "14 L FIG. l4

W g l4 I2 il -|O0 k 58 52 62 *E a J 7 P 2 60 J 1| V 156 64 RADAR MOUNTED ON HELICOPTER BACKGROUND OF THE INVENTION This invention relates to a radar apparatus mounted on a helicopter and more particularly to improvements in the mounting of an antenna for such a radar apparatus and the associated components.

Conventional antennas for radar apparatus mounted on helicopters can be divided broadly into the following two classes in terms of their mounting to the plane or fuselage of the helicopter:

One of the two classes called A hereinafter includes antennas mounted on one portion of the fuselage of helicopters and the other class called B hereinafter includes antennas embedded in the rotor blade of helicopters. In the class A of antennas there are known antennas of the types mounted to the nose cone, to the rear side of the fuselage and disposed on the sponson of the helicopter while in the class B there are known antennas of thetype including elongated antenna elements embedded in the rotor blades of'the helicopter.

In general, radar apparatus equipped on helicopters are very much limited in the dimension and weight of components forming the apparatus and therefore a serious problem arises in how the radar apparatus is formed to satisfy the required performances with the maximum permissible dimension and weight thereof. Among those performances a representative one is the radar range or the maximum distance at which the radar apparatus is ordinarily effective in detecting objects. It is well known that the greater the effective area of the associated antenna the longer the radar range will be with other conditions remainingunchanged. On the other hand, those performances considered to be important from the operational point of view involve the coverage, the data rate etc. Namely, the coverage should be sufficiently broad and the data rate is required to be high enough to permit radar echoes from relatively high speed targets to be displayed continuously on the associated display device with a high brightness.

The prior art practice will now be discussed in terms of the above cited performances. First, in the class A antennas, they have been considerably limited in the effective area because the latter greatly affects the flying performances of the helicopter. As a result, the long radar range has been difficult to be obtained. In addition, the antenna has been partly shaded with the rear fuselage portion where the antenna is mounted limiting the searching area or coverage thereof. Also one means for increasing the data rate is to rotate the antenna at a high speed. Since those antennas belonging to class A are adapted to be driven by their own driving mechanisms, an increase in speed of rotation of the antenna requires to make the driving mechanism large-scaled. Therefore due to the limitations as to the dimension and weight, it has been difficult to make the mechanism large-scaled and even if possible, the resulting mechanism would be low in reliability.

On the other hand, in the class B of antennas, they are arranged to be driven by the same driving mechanism as do the associated rotor blades of helicopters. This eliminates the necessity of providing a separate driving mechanism for exclusively driving the antenna and results in a corresponding decrease in the total weight and hence the required electric power. Also the resulting reliability is increased. On the other hand, the antenna has its vertical dimension impossible to be greater than the thickness of the associated rotor blade and its horizontal dimension is inhibited from being much larger due to the mechanical strength of that blade. Thus the resulting antennas have been impossible to be sufficiently large in the effective area and therefore they have been considerably limited in radar range. Further that antenna embedded in the rotor blade has been difficult to be manufactured and its maintenance is impeded.

The occurrence of any fault in radar-apparatus mounted to helicopters will cause a serious obstacle to the safe flight thereof. Thus if one lays much stress on the safety then it is desirable to dualize the radar apparatus so that with one of the apparatus out of order, the other apparatus can be utilized to ensure safety.

Further radar apparatus mounted on helicopters have normally the function of surveying the nature of terrain around the helicopter and searching obstacles existing in the air and on the surface of the sea. However, the radar apparatus may be desirable, in adddition to the abovementioned function, to have another function of receiving response wave from any consort provided with the associated responder to locate it. Normally, those radar apparatus having the firstmentioned function are called the primary radar and those having the second function are called the secondary radar". In order to take greatly advantage of both functions, it is desirable to differentiate radar frequency for use with a primary radar from that for a secondary radar in accordance with the function thereof.

SUMMARY OF THE INVENTION Accordingly it is an object of the invention to provide a new and improved radar apparatus for use with a helicopter capable of sufficiently increasing a maximum detectable range, sufficiently broadening a coverage to be searched and increasing a data rate with a construction simple and easy to be manufactured.

It is another object of the invention to provide a radar apparatus of the type as described in the preceding paragraph formed into the dual type.

It is still another object of the invention to provide a radar apparatus of the type as described in the preceding paragraph operative satisfactorily as the primary and secondary radars.

The invention accomplishes these objects by the provision of a radar apparatus for use with a helicopter having rotor means including a rotary shaft, a rotor head disposed on the upper end of the rotary shaft, and a plurality blades radially extending at predetermined substantially equal angular intervals from the rotor head, the radar apparatus comprising an antenna system secured upon the rotor head so as to have a maximum directive gain between a selected pair of adjacent blades, the antenna system being rotatable with the rotor means, and a transmitter/receiver unit coupled to the antenna system to supply a transmission power to the latter and receive radar signals picked up by the antenna system.

In a preferred embodiment of the invention, the antenna may be disposed within a radar dome in the form of an oblate spheroid and connected to the transmitter/receiver unit through a transmission line extending through the rotary shaft and a rotary joint disposed at the lower end of the rotary shaft to electrically couple the antenna system to the transmitter/receiver unit therethrough.

In order to form the radar apparatus into the dual type or to operate it as the primary and secondary radars, a pair of antenna similar or dissimilar to each other may be secured upon the rotor head so that the maximum directive gains thereof are directed to between different pairs of adjacent blades respectively. the antennas being rotable with the rotor means. and a pair of first and second transmitter/receiver units are coupled to the pair of antennas respectively.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will become more readily apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. I is a combined plan and block diagram of a radar apparatus constructed in accordance with the principles of the invention with the associated helicopter illustrated fragmentarily;

FIG. 2 is an elevational view of the antenna system shown in phantom in FIG. 1 with one half of the radar dome removed for illustrating the details of the antenna system;

FIG. 3 is a plan view of components disposed around the antenna system;

FIG. 4 is a vertical radiation field pattern exhibited by the antenna system shown in FIGS. 1 through 3;

FIG. 5 is a side elevational view, partly in section of an annular waveguide rotary joint schematically shown in FIG. 1;

FIG. 6 is a view similar to FIG. 1 but illustrating a modification of the invention;

FIG. 7 is a view similar to FIG. 2 but illustrating the modification of the invention shown in FIG. 6;

FIG. 8 is a view similar to FIG. 3 but illustrating the modification of the invention shown in FIG. 6;

FIGS. 9 and 10 are views similar to FIGS. 4 and 5 but relating to the modification of the invention shown in FIG. 6; and

FIGS. 11 through 14 are views similar to FIGS. 1 through 5 respectively but illustrating another modification of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and FIG. 1 in particular, there is illustrated a radar apparatus constructed in accordance with the principles of the invention and mounted on a helicopter that is partly shown in FIG. 1. The arrangement illustrated comprises a helicopter including a plane or fuselage a part of which is designated by the reference numeral 10 and a rotary shaft 12 rotatably supported by bearings (not shown) to substantially vertically extend through the central portion of the fuselage 10 until the upper end thereof projects beyond the upper surface of the helicopter. The rotary shaft 12 is of a hollow type having a central bore 14 extending therethrough. In order to rotate the shaft 10 about its axis, a gearing 16 is mechanically connected to the intermediate portion of the shaft 12 around the outer periphery and adapted to be driven by a driving mechanism (not shown).

As shown in FIG. 1, a rotor head 18 is fixedly secured to the upper end of the rotary shaft 12 and has a plurality of rotor blades 20 radially extending at substantially equal angular intervals from the same to form a rotor generally designated by the reference numeral 21.

The arrangement further comprises an antenna assembly generally designated by the reference numeral 22 and fixed to the upper surface of the rotor head 18 to be rotatable with the rotary shaft 12.

Referring now to FIG. 2, the antenna assembly 22 includes a housing or a radar dome 24 in the form of an oblate spheroid. a disc-shaped base member 26 disposed on the central bottom portion of the radar dome 22, a plurality of fitting legs 28 extending from the peripheral portion of the base member 26 externally of the radar dome 21, and a strut 30 formed integrally with the base member 26 to extend centrally throughout the height of the radar dome until the upper end thereof is attached to the top of the dome. The antenna assembly 22 is rigidly secured to the rotor head 18 through the fitting legs 28. The central axis of the strut 30 is vertically aligned with that of the rotary shaft 12.

The strut 30 has a reflecting mirror 32 disposed on one side, in this example, the lefthanded side as viewed in FIG. 2 thereof and a counterweight 34 disposed on the opposite side thereof in diagramatically opposite relationship with the mirror 32. The counterweight 34 serves to impart to the strut 30 a weight counterbalancing the weight of the mirror 32 with respect to the longitudinal axis of the strut 32 ensuring the smooth rotation of the strut 30 and therefore of the rotary shaft 12. The reflector or mirror 32 along with a horn or a radiator 38 forms an antenna system generally designated by the reference numeral 40.

A supporting arm 36 is disposed integrally with the lower portion of the mirror 32 so as to extend therefrom toward the adjacent end of the radar dome 24. The arm 36 supports a horn 38. More specifically, the horn 38 fixedly extending through the supporting arm 36 has one end in the form of a flare opening at a point where the reflecting surface of the mirror 32 has a focus and the other end portion projecting beyond the adjacent portions of the base member and radar dome 26 and 24 respectively. The supporting arm 36 integral with the mirror 32 ensures that the mirror 32 is prevented from being displaced with respect to the horn 38 due to the vibration of the fuselage 10 (see FIG. 1

As above described, the radar dome 24 has the base member 26, the strut 30, the antenna system 40 disposed in place therein and serves to protect the antenna assembly 22 while decreasing the resistance presented to an air stream flowing around the same because it is in the form of an oblate spheroid.

FIG. 3 shows the positional relationship between the antenna system 40 and the blades 20 of the rotor 21. While the rotor 21 is shown in FIG. 3 as including six blades 20, it is to be understood that the rotor may include any desired number of the blades 20. The six rotor blades 20 radially extend at substantially equal angular intervals equal to an angle of about the center of the antenna assembly 22. It is important that, with the best result, the rotor blades 20 should be fixedly disposed in such a manner that a line bisecting an angle formed of a selected pair of the longitudinal axes of adjacent blades 20 at the center of the antenna assembly 22 is in the same direction as does a line interconnecting the center and focus of the reflecting surface of the mirror 32, that is to say, a line in which the antenna system 40 has a maximum directive gain. The latter line is designated at the arrow 39 in FIG. 3. The

positional relationship between the rotor blades and the antenna system 40 as above described is important because a microwave is permitted to be transmitted or picked up by the antenna system 40 without any interference with the rotor blades 20.

The direction of the maximum directive gain of the antenna system relative to the rotor blades as above described is not absolutely necessary and it has been found that the antenna system 40 is sufficient to have its maximum directive gain between a selected pair of adjacent rotor blades. That is. the direction of the maximum directive gain is permitted to somewhat deviate from the abovementioned bisecting line on one or the other of the sides whereby the rotor blades 20 are prevented from interfering with the antenna system 40.

In the arrangement of FIG. 1 the antenna system 40 is operative as an antenna for use with the primary radar having the function of surveying the nature of terrain around the associated helicopter and searching obstacles existing both in the air and on the sea. Therefore the antenna system 40 is designed and constructed such that it has radiation field pattern suitable for performing that function. The radiation field pattern can be generally divided into a vertical and a horizontal pattern. The antenna system 40 has preferably a vertical pattern as shown in FIG. 4.

In FIG. 4 the reference character 0 designates the center of the reflecting surface of the mirror 32 forming a part of the antenna system 40, and line I I designates the horizon passing through the center 0. Curve C depicts a vertical pattern in a vertical plane containing the line interconnecting the center and focus of the reflecting mirror surface forming a part of the antenna system 40. That is, curve C has been drawn on the basis of data obtained by measuring a field strength provided by a microwave transmitted by the antenna system 40 in a direction having an angle of elevation of 0 at the point 0 while varying the angle of 0. The angle of 0 is positive and negative above and below the horizon respectively. The field strength has a maximum value at a point a in the horizon 1 1 and l/w/ 2 or 70.7% of the maximum value at a point b in a line passing through the point 0 to form a negative angle of about 7 with the horizon. Then as the angle of 6 negatively increases, the field strength gradually decreases until it reaches a point c from which the field strength begins to abruptly decrease:

That portion of curve C extending from the point 0 through the point a to the point b can be expressed by cos 6 and represents an antenna pattern normally called a fan beam. As the rotary shaft 12 of the helicopter (FIG. 1) is tilted with respect to the horizon during the flight thereof, the antenna system 40 can have an angle of elevation directed downwardly in the advancing direction and upwardly in the opposite direction. Even under these circumstances, that antenna pattern representative of the curve portion O-a-b is effective for sensing any obstacle existing in the air at a level identical to or higher than the level of the associated helicopter. That portion of curve C extending between the points b and 0 can be expressed by cosec 0 and represents an antenna pattern effective for surveying the nature of terrian and searching targets on the sea.

The antenna system 40 has a radiation field pattern in a horizontal plane including the same or a horizontal pattern in the form ofa narrow fan beam having a beam width of about 2 in order to increase its horizontal resolution. Thus the antenna system 40 is of a relatively narrow fan beam type.

Referring back to FIG. 1, the horn 38 has a horizontal polarization and is bent in the H plane for the waveguide. That end opposite to the flared open end of the horn 38 is coupled through a section of flexible waveguide 42 toa waveguide I-I corner 44 in order to compensate for any relative transverse displacement between the horn and H corner 38 and 44 respectively due to vibrational movements thereof. The H-corner 44 is bent into the central bore 14 in the rotary shaft 12 and connected to a section of waveguide 46 for supplying a microwave energy to the horn 38. The section of waveguide 46 extends through the central bore 14 until its lower end as viewed in FIG. 1 is coupled to a section of twistable waveguide 48 disposed on the lower end portion of the bore 14. Disposed directly below the lowerend of the bore 14 in the rotary shaft 12 is a waveguide rotary joint 50 in the form of an annulus connected to the section of twistable waveguide 48. The purpose of the twistable waveguide 48 is to provide an allowance for an angle at which the waveguide 46 is coupled to the annular rotary joint 50 as well as preventing any vibration of the waveguide section 46 from being transferred to the waveguide rotary joint 50.

The annular waveguide rotary joint 50 is of a well known construction such as shown in more detail in FIG. 5. The rotary joint 50 illustrated comprises a rotatable member 52 in the form of an annulus snugly fitted into the central bore 12 in the rotary shaft 12, a stationary member 54 in the form of an annulus suitably secured to a fixed member (not shown), and a bearing 56 disposed between the rotatable and stationary members 52 and 54 respectively to permit the relative rotational movement therebetween. It will be noted that the rotatable and stationary members 52 and 54 respectively have the respective longitudinal axes aligned with each other and also with the longitudinal axis of the rotary shaft 12, and that both members 52 and 54 form a very small clearance between the adjacent surfaces thereof in the direction of the aligned longitudinal axes as above described. Those adjacent surfaces are provided with respective annular grooves 58 and 60 in opposite relationship. Either one of the grooves 58 and 60 includes a grid (not shown) in the manner as well known in the art.

The annular groove 58 on the rotatable member 52 is coupled to the section of twistable waveguide 48 (not shown in FIG. 5) through a section of waveguide 62 while the annular groove 60 on the stationary member 54 is coupled to a section of waveguide 64. The grooves 58 and 60 have respective dimensions equal to the inside diameters of the associated waveguides. A microwave energy is adapted to be transferred from one to the other of the annular grooves 58 and 60.

Again referring back to FIG. 1, the section of waveguide 64 is coupled to a section of waveguide 66 subsequently connected through a flexible waveguide section 68 to a transmitter/receiver unit 70 which may be disposed at any desired position in the fuselage 10. It will readily be understood that a transmission line disposed between the waveguide 64 and the transmitter/- receiver unit 70 may be suitably varied in accordance with'the position of the unit 70. The flexible waveguide section 68 included in the transmission line just described is effective for acting as a mechanical buffer.

In order to enable the transmitter/receiver unit 70 to effectively perform the function as the primary radar system, the unit 70 may be designed and constructed such that it transmits and receives a microwave having a frequency in the Ku band normally ranging from 12.5 to 18.0 gigahertz. That is, the unit may be set to operate at a frequency within the Ku band. In this connection. it is noted that the higher the operating frequency the higher the resolution of bearing will be with an equipment smaller in dimension and lighter in weight.

A microwave energy generated by the transmitter/- receiver unit 70 passes through the flexible waveguide 68, the waveguide 66, the waveguide rotary joint 50, the twistable waveguide 48, the waveguide 46, the H- corner 44 and, the flexible waveguide 42 in the named order and then it is radiated into the air by the antenna system 40. A radar echo reflected from any target and picked up by the antenna system 40 is returned back to the transmitter/receiver unit 70 through the abovementioned components in the order reversed from that above described for the radiation. In the unit 70, the received radar echo is amplified and detected to form a received signal.

In order to display the received signal, the arrangement of FIG. 1 comprises a plan position indicator (PPI) 72. The indicator 72 is disposed adjacent a pilots seat (not shown) in the helicopter along with a control box 74 for controlling both the transmitter/receiver unit 70 and the indicator 72 thereby to control the entirety of the radar apparatus. To this end, the control box 74 is connected to both the transmitter/receiver unit 70 and the indicator 72 through a length of electric cable 76 which is, in turn, connected to a source of electric power (not shown).

As shown in FIG. 1, a synchronous resolver 78 of the well known construction is operatively coupled to the rotary shaft 12. More specifically, the resolver 78 includes a rotor (not shown) mechanically connected to the rotary shaft 12 as shown at broken line with a gear ratio of 1 1 and is operative to generate a pair of signals in the quadrature having a frequency of 400 hertz for example and equal amplitudes varied in a sinusoidal manner in accordance with an angle through which the rotor is rotated with respect to a stator involved thereby to provide a measure of an angular position of the rotary shaft relative to a reference. The synchronous resolver 78 cooperates with both the transmitter/- receiver unit 70 and the antenna system 40 to form a radar system.

The pair of signals thus generated are applied by a length of electric cable 80 to the indicator 72 as a bearing signal representative of an instantaneous direction in which the antenna system 40 has the maximum directive gain. In the indicator 70 that bearing signal cooperates with the signal from the transmitter/receiver unit 70 to display the plan position indicator trace.

The arrangement as above described is effective for permitting the effective area of the antenna system 40 to be sufficiently large without much impeding the flying performances of the associated helicopter by the antenna assembly 22 disposed upon the rotor head 18 and including the antenna system 40 therein. This results in an increase in the maximum detectable range of the radar. Also the direction of the maximum directive gain of the antenna system 40 is between a selected pair of adjacent rotor blades 20 to permit the antenna system to search every direction about the axis of the rotary shaft 12 while the blades are prevented from impeding the performances of the radar apparatus. This is effective for sufficiently broadening the coverage of the radar apparatus. Furthermore the antenna system 40 is driven by the rotary shaft 12 which eliminates the necessity of coupling the system to its own drive means and permits the antenna system to be driven at a high speed. Therefore the data rate is possible to be high in a simple manner.

The arrangement as shown in FIGS. 1 through 5 can be modified to be formed into a dual structure such as shown in FIGS. 6 through 10 wherein like reference numerals designate the components identical or corresponding to those shown in FIGS. 1 through 5. Also those components added to the arrangement for the purpose of forming the dual structure are identified by the same reference numerals as designating the corresponding components suffixed with the reference character A. For example, the reference numeral 36 suffixed with A or 36A designates a supporting arm rigidly secured to the reflecting mirror 32A to support the antenna horn 38A.

With the rotor 21 including six blades 20 as shown in FIG. 8, a pair of antenna systems 40 and 40A identical to each other and to the system 40 as best shown in FIG. 2 are disposed in diametrically opposite relationship with each other and in symmetry with respect to the axis of the strut 30 in the same manner as above described in conjunction with FIG. 2 within the radar dome 24 (see FIG. 7). That is, the reflecting mirror 32A is substituted for the counterweight 34 as shown in FIG. 2 and also has the same purpose as the counterweight 34. Then both antenna systems 40 and 40A are electrically connected to their own transmitter/receiver units and 70A through individual transmission lines respectively. Both transmitter/receiver units are identical to each other and may be of a conventional construction. Except for the annular waveguide rotary joints 50 and 50A, each of the individual transmission lines is formed of various sections of waveguide identical to those as above described in conjunction with FIGS. 1 and 2. Both transmission lines are preferably disposed symmetrically with respect to the axis of rotation of the rotary shaft 12 within the central bore 14.

The direction of the maximum directive gain of one of the antenna systems 40 or 40A is shifted from that of the other antenna systems 40A or 40 by a predetermined angular interval a, and lies in a line bisecting an angle of formed of a selected pair of adjacent rotor blades 20. The other antenna system has its maximum directive gain lying in a line bisecting an angle formed of another selected pair of adjacent blades 20. In FIG. 8, the predetermined angular interval is shown as being equal to an angle of and both bisecting lines 39 and 39A come into line. With the rotor including five blades, the angle a is of about 144.

Referring back to FIG. 6, a pair of synchronous resolvers 78 and 78A identical in construction to each other are mechanically coupled to the rotary shaft 12 in the same manner as does the resolver 70 shown in FIG. 1 to provide bearing signals for both antenna systems 40 and 40A. The synchronous resolvers 78 and 78A maybe any of conventional constructions but are designed and constructed such that due to the predetermined angular interval a between the directions of the maximum directive gain of both antenna systems 40 and 40A. the outputs therefrom should have a predetermined phase difference equal to that angular interval a, in this case, an angle of l80 therebetween.

The bearing signals from both resolvers 78 and 78A are applied through respective lengths of electric cables 80 and 80A to a common plan position indicator (PPI)72 such as shown by the reference numeral 72 in FIG. I. As in the arrangement of FIG. I, the control box 74 is provided for controlling both the indicator 72 and the pair of transmitter/receiver units 70 and 70A through lengths of electric cable 76 and 76A.

In FIG. 9 wherein like reference numerals designate the components identical to those shown in FIG. 4, a vertical radiation field pattern for the combination of both antenna systems 40 and 40A is shown as including a vertical radiation field pattern identical to that shown at curve C40 in FIG. 4 and having another pattern C identical to the pattern C and arranged in symmetry with respect to a vertical line I [1' with the point common to both patterns. The pattern C results from the antenna system 40 while the pattern C is provided by the antenna system 40A.

FIG. wherein like reference numerals designate the components identical or simmilar to those shown in FIG. 5 illustrates the annular waveguide rotary joints 50 and 50A formed together into a unitary structure with certain parts common to both joints for the purpose of compactness. As shown in FIG. 10, the annular rotatable members 52 and 52A for the two antenna systems 40 and 40A are formed into a single annular piece having one end or the upper end snugly fitted into the central or axial bore 14 in the rotary shaft 12. A pair of annular stationary members 54 and 54A are disposed in opposite relationship to rotatably embrace the annular rotatable piece 52 52A through the bearing 56. Each end face of the annular rotatable piece 52 52A opposes to the annular stationary member 54 and 54A to form a very small clearance therebetween. As in the arrangement of FIG. 5, a pair of annular grooves 58 and 60 are disposed in opposite relationship on the opposing surfaces of the upper rotatable portion and stationary members 52 and 54 respectively. Similarly, another pair of annular grooves 58A and 60A are disposed in opposite relationship on the lower rotatable portion and stationary member 52A and 54A respectively. Then the annular grooves 58 and 58A are coupled to sections of waveguide 62 and 62A respectively while the annular grooves 60 and 60A are coupled to sections of waveguide 64 and 64A respectively.

Then the sections of waveguide 62 and 62A are coupled to the horns 38 and 38A through their own transmission lines and the sections of waveguide 64 and 64A are coupled to the transmitter/receiver units 70 and 70A through the sections of waveguides 66, 68 and 66A and 68A respectively as shown in FIG. 6.

In the arrangement as above described in conjunction with FIGS. 6 through 10, the antenna system 38, transmitter/receiver unit 70 and synchronous resolver 78 form a first radar sytem while the corresponding components 38A, 70A and 78A form a second radar system with the indicator 72 common to both radar systems. Normally only either one of the radar systems, for example. the first one is arranged to be always put in operation while the other or secondradar system is maintained inoperative with the output from the second synchronous resolver 78A disenabled. Under these circumstances, if the first radar system in operation becomes out of order, the searching or surveying operation will be immediately taken over by the second radar system with the transmitter/receiver unit A put in operation and with the second resolver providing the bearing information.

FIGS. 11 through 14 wherein like reference numerals designate the components identical or corresponding to those shown in FIGS. 1 through 5 illustrates another modification of the invention wherein a secondary radar system is incooporated into a primary radar system as shown in FIGS. 1 through 5. As best shown in FIG. 12, the strut 30 has rigidly secured thereto a selective identification feature (SIF) interrogation antenna system 90 in place of the counterweight 34 as shown in FIG. 2. The interrogation antenna system 90 is different in type from the associated antenna system 40. In this connection, it is desirable to differentiate the frequency bands for transmission and reception between both antenna systems 40 and 90. The interrogation antenna system is preferably formed of a four-element antenna array including four loop antennas (not shown) adapted to form a fan-shaped radar beam. That is, the antenna system 90 has a horizontal directive pattern relatively narrow in beam width for the purpose of increasing the resolution of bearing for consorts and a vertical "pattern broad in beam width enough to substantially eliminate the effect of any change in angle of elevation of the antenna system 90 due to inclination of the associated helicopter during the flight thereof.

The antenna system 90 has a maximum value of its directive gain in the direction of the arrow 91 shown in FIG. 13. That is, the maximum directive gain appears in a line besecting an angle formed of a selected pair of adjacent rotor blades 20, in this case, the two blades 20 disposed above and below the arrow 91. On the other hand, the antenna system 40 has a maximum value of its directive gain in the direction of the arrow 39 as shown in FIG. 13 in which a line extends to bisect an angle formed of another selected pair of adjacent blades 20. With the rotor 21 including six blades 20 as shown in FIG. 13, the direction of the arrow 39 is diametrically opposite to the direction of the arrow 91 to form an angle [3 equal to I therebetween. If the rotor includes five blades, that angle ,8 is of about 144.

As shown in FIG. 11, the antenna system is coupled to a coaxial rotary joint 92, through a section of waveguide extending through the central or axial bore 14 in the rotary shaft 12. As best shown in FIG. 14, the coaxial rotary joint 92 centrally extends through the hollow portion of the waveguide rotary joint 50 in electrically insulated relationships with the joint 50. The joint 92 may be of a miniature contact type and includes a rotatable portion 94 rotatably fitted into a stationary portion 96 adapted to effect a relative rotation therebetween with both members maintained in mechanical contact with each other. The stationary portion 96 is rigidly secured to and extends through a supporting disc 98 subsequently fixed to the stationary member 54 of the annular rotary joint 50. The rotatable and stationary portions 92 and 94 respectively have a common central axis coinciding with that of the rotary shaft 12. In other respects, the arrangement is identical to that shown in FIG. 5.

The coaxial rotary joint 92 and strictly its stationary portion 96 is connected by a length of coaxial cable 102 toan SIF interrogation transmitter/receiver unit 104 well known in the art (see FIG. 11) and connected to both the indicator and control box 72 and 74 respectively through a length of an electric cable 106.

The transmitter/receiver unit 104 is different in operating frequency from the transmitter/receiver unit 70. With the transmitter/receiver unit 70 operated in the Ku band as above described, the unit 104 is preferably operated in the L-band ranging from 1000 to I100 megahertz.

As in the dual type arrangement above described, the rotary shaft 12 has, in addition to the synchronous resolver 78, another synchronous resolver 108 operatively coupled thereto in the same manner as above described in conjunction with the resolver 78A to provide the information concerning the bearing of the interrogation antenna system 90. Such information is applied to the indicator 72 through a length of electric cable 110. That information is phase shifted from that provided by the resolver 78 by the abovementioned angle B between the directions of the maximum directive gain for both antenna systems 40 and 90.

In other respects, the arrangement is identical to that shown in FIGS. 1 through 5.

In operation, a pair of microwave pulses (mode pulses) generated by the interrogation transmitter/- receiver unit 104 is supplied to the antenna system 90 through the length of coaxial cable 102, the coaxial joint 92 and the length of coaxial cable 100 in the named order and thence to the antenna system 90 which, in turn, radiates it into the air. A replied SIF coded signal from any consort provided with the associated responder is picked up by the antenna system 90. This signal is supplied to the transmitter/receiver unit 104 through the abovementioned transmission line in the order reversed from that for transmission. The unit 104 decodes the applied signal to form a single pulse concerning the presense of the consort to be applied to the indicator 72.

In the plan position indicator 72, the outputs from both synchronous resolvers 78 and 108 are repeatedly and alternately enabled in synchronization with the corresponding switching of transmission from one of the primar radar and interrogation units to the other whereby the indicator 72 has indicated thereon a corresponding spot in a radial direction coinciding with that direction in which the corresponding antenna system has the maximum directive gain at that time and at a distance as determined by the time interval between the transmission and reception for that signal. In this way, the PPI-display is effected on the indicator 72.

In the arrangement shown in FIGS. 11 through 14 it is noted that the primary radar pulse to be transmitted is generated each time a predetermined time interval equal to the sum of the duration of the interrogating pulse, say, about 8 microseconds, the duration of the response pulse from the associated responder, say, about microsecond and a time delay occuring in the responder, say, about 3 microseconds, has been timed out starting with the main triggering for generating the corresponding interrogating pulse. With the just specified figures for the durations and time delay the time interval amounts at about 36 microseconds, but it is to be understood that the invention should not be restricted to the figures as above specified. Under these circumstances, any primary radar echo and any SIF response signal identical in both bearing and range to each other is displayed a single spot on the indicator 72.

From the foregoing, it will be appreciated the objects of the invention are accomplished by the provision of an antenna system disposed upon a rotor head on a helicopter for rotation with a rotor involved so that the antenna system is maximum in directive gain in a line bisecting an angle formed ofa selected pair of adjacent placed provided on the rotor. The antenna system may be constructed into a dual structure. Also the secon-' dary radar system may be incorporated into the present antenna system.

While the invention has been illustrated and described in conjunction with a few preferred embodiments thereof, it is to be understood that numerous changes and modifications may be resorted to without departing from the spirit and scope of the invention. For example, in the arrangement as shown in FIGS. 6 through 10, the first and second radar systems may be continuously and simultaneously put in operation. To this end, the indicator 72 may be operative to utilize repeatedly and alternately the two sets of bearing signals from the synchronous resolvers and 70A in synchronization with the switching of transmission from one to the other of the transmitter/receiver units 70 and 70A. Alternatively, the indicator 72 may be provided with a dual gun type cathode ray tube in which each of electric guns involved is adapted to be continuously applied with the bearing information from the associated synchronous resolver: Further in the arrangement as shown in FIGS. 11 through 14, only the primary radar echo or the associated SIF response signal may be displayed on the indicator 72 at one time, instead of being displayed at the same time.

What we claim is:

l. A radar apparatus for use with a helicopter having rotor means including a rotary shaft, a rotor head disposed upon the upper end of said rotary shaft, and a plurality of rotor blades radially extending at predetermined substantially equal angular intervals from said rotor head and defining a plane of rotation, said apparatus comprising an antenna system secured on said rotor head above the level of the rotor blades and disposed to have a direction of maximum gain between a selected pair of said adjacent rotor blades and substantially within the plane of rotation, said antenna system being rotatable with said rotor means, an oblate spheroidal radar dome, means mounting said radar dome with said antenna system disposed therein on said rotor head, a transmitter/receiver unit coupled to said antenna system to supply a transmission power to the latter and receive radar signals picked up by said antenna system, means coaxial with said rotary shaft connecting said antenna and said transmitter/receiver, and a rotary joint disposed at the rotor head to connect said antenna system to the lastmentioned means.

2. A radar apparatus as claimed in claim 1 further comprising a second antenna system disposed within said radar dome so as to have a direction of maximum gain between different pairs of adjacent rotor blades respectively and substantially within the plane of rotation, a pair of transmitter/receiver units comprising the first-mentioned transmitter/receiver coupled to the first-mentioned and second antenna systems respectively and each having means supplying transmission power to the associated transmitter/receiver unit and means to receive radar signals picked up by the associated antenna system, said first rotary joint and a second rotary joint disposed at the lower end of said rotor shaft to electrically couple said first-mentioned and said second transmitter/receiver units to the associated antenna systems respectively, said means coaxial with said rotor shaft comprising a pair of first and second transmission lines extending through the interior of said rotor shaft to couple said first-mentioned and said second rotary joints to the associated antenna system respectively, angle detector means coupled to said rotor shaft to generate signals for angular information corresponding to the maximum directive gain of said firstmentioned and second antenna systems respectively and display means for receiving both the radar signals from each of said transmitter/receiver units and the signals for angular information from said angle detector means to display a position of a target.

3. A radar apparatus as claimed in claim 1, wherein said radar dome includes a strut rotatable with said rotor shaft and wherein said strut has secured thereto said antenna system and includes a counter weight to balance said antenna system.

4. A radar apparatus for use with a helicopter having rotor means including a rotor shaft, a rotor head disposed upon the upper end of said rotor shaft, and a plurality of rotor blades radially extending at predetermined substantially equal angular intervals from said rotor head and defining a plane of rotation, said radar apparatus comprising a pair of first and second antenna systems secured upon said rotor head so as to be rotatable with said rotor shaft, said first and said second antenna systems each having a direction of maximum gain and each being disposed having the respective directions of maximum gain between different pairs of said adjacent rotor blades and disposed subtantially within the plane of rotation, an oblate spheroidal radar dome, means mounting said radar dome on said rotor head with said antenna systems disposed therein, a pair of transmitter/receiver units, rotary joint means at the lower end of said rotary shaft to electrically couple said antenna systems to said transmitter/receiver units, transmission line means extending through theinterior of said rotary shaft to electrically connect said antenna systems to said transmitter/receiver units, angle detector means coupled to said rotary shaft to generate a signal for angular information corresponding to the maximum directive gains of said antenna systems, and display means for receiving both radar signals from said antenna systems and said signal for angular information from said angular detector means to display a position of a target.

. 5. A radar apparatus as claimed in claim 4 wherein said rotary joint means comprises a first rotary joint comprising an angular waveguide rotary joint and a second rotary joint centrally disposed therein.

6. A radar apparatus as claimed in claim 5 wherein said transmitter/receiver units comprises a first transmitter/receiver unit for transmitting a signal within the Ku band of frequencies to perform the function of a primary radar and a second transmitter/receiver unit for transmitting a signal within the L band of frequency to perform the function of a secondary radar. 

1. A radar apparatus for use with a helicopter having rotor means including a rotary shaft, a rotor head disposed upon the upper end of said rotary shaft, and a plurality of rotor blades radially extending at predetermined substantially equal angular intervals from said rotor head and defining a plane of rotation, said apparatus comprising an antenna system secured on said rotor head above the level of the rotor blades and disposed to have a direction of maximum gain between a selected pair of said adjacent rotor blades and substantially within the plane of rotation, said antenna system being rotatable with said rotor means, an oblate spheroidal radar dome, means mounting said radar dome with said antenna system disposed therein on said rotor head, a transmitter/receiver unit coupled to said antenna system to supply a transmission power to the latter and receive radar signals picked up by said antenna system, means coaxial with said rotary shaft connecting said antenna and said transmitter/receiver, and a rotary joint disposed at the rotor head to connect said antenna system to the lastmentioned means.
 2. A radar apparatus as claimed in claim 1 further comprising a second antenna system disposed within said radar dome so as to have a direction of maximum gain between different pairs of adjacent rotor blades respectively and substantially within the plane of rotation, a pair of transmitter/receiver units comprising the first-mentioned transmitter/receiver coupled to the first-mentioned and second antenna systems respectively and each having means supplying transmission power to the associated transmitter/receiver unit and means to receive radar signals picked up by the associated antenna system, said first rotary joint and a second rotary joint disposed at the lower end of said rotor shaft to electrically couple said first-mentioned and said second transmitter/receiver units to the associated antenna systems respectively, said means coaxial with said rotor shaft comprising a pair of first and second transmission lines extending through the interior of said rotor shaft to couple said first-mentioned and said second rotary joints to the associated antenna system respectively, angle detector means coupled to said rotor shaft to generate signals for angular information corresponding to the maximum directive gain of said fiRst-mentioned and second antenna systems respectively and display means for receiving both the radar signals from each of said transmitter/receiver units and the signals for angular information from said angle detector means to display a position of a target.
 3. A radar apparatus as claimed in claim 1, wherein said radar dome includes a strut rotatable with said rotor shaft and wherein said strut has secured thereto said antenna system and includes a counter weight to balance said antenna system.
 4. A radar apparatus for use with a helicopter having rotor means including a rotor shaft, a rotor head disposed upon the upper end of said rotor shaft, and a plurality of rotor blades radially extending at predetermined substantially equal angular intervals from said rotor head and defining a plane of rotation, said radar apparatus comprising a pair of first and second antenna systems secured upon said rotor head so as to be rotatable with said rotor shaft, said first and said second antenna systems each having a direction of maximum gain and each being disposed having the respective directions of maximum gain between different pairs of said adjacent rotor blades and disposed subtantially within the plane of rotation, an oblate spheroidal radar dome, means mounting said radar dome on said rotor head with said antenna systems disposed therein, a pair of transmitter/receiver units, rotary joint means at the lower end of said rotary shaft to electrically couple said antenna systems to said transmitter/receiver units, transmission line means extending through the interior of said rotary shaft to electrically connect said antenna systems to said transmitter/receiver units, angle detector means coupled to said rotary shaft to generate a signal for angular information corresponding to the maximum directive gains of said antenna systems, and display means for receiving both radar signals from said antenna systems and said signal for angular information from said angular detector means to display a position of a target.
 5. A radar apparatus as claimed in claim 4 wherein said rotary joint means comprises a first rotary joint comprising an angular waveguide rotary joint and a second rotary joint centrally disposed therein.
 6. A radar apparatus as claimed in claim 5 wherein said transmitter/receiver units comprises a first transmitter/receiver unit for transmitting a signal within the Ku band of frequencies to perform the function of a primary radar and a second transmitter/receiver unit for transmitting a signal within the L band of frequency to perform the function of a secondary radar. 